Solid-state amplifiers include a mode locked-laser delivering seed-pulses to be amplified; a pulse-stretcher for temporally stretching the seed-pulses; an amplifier for amplifying the temporally stretched pulses; and a pulse-compressor for temporally compressing the amplified pulses. The amplifier may be a regenerative amplifier, which includes a resonator including an optical switch arranged to accept a pulse to be amplified then discharge the amplified pulse after a predetermined number of round trips in the resonator. The amplifier may also be a multi-pass amplifier wherein the beam is sent many times through the same amplifying medium by using small angular separations between passes. Typically the pulse-stretcher and the pulse-compressor include either a single reflective diffractive grating (grating) or a pair of reflection-gratings, with an optical path for the pulses being stretched or compressed making multiple non-normal incidences on the grating or gratings.
The primary mechanism for providing temporal stretching and temporal compression of pulses is group delay dispersion (GDD), which is the second derivative of the reflected phase. GDD is measured in units of femtoseconds squared (fs2). Pulse-stretching involves adding positive GDD. Correspondingly, negative GDD provides pulse-compression. As output pulses become shorter, pulse-shape becomes dependent of third-order dispersion (TOD), and eventually fourth-order dispersion (FOD). TOD is measured in units of femtoseconds cubed (fs3). FOD is measure in units of femto seconds to the power four (fs4).
By way of example, for pulses having a duration of about 1000 femtoseconds (fs), only GDD need be controlled. For pulses having a duration of 100 fs, GDD and TOD need to be controlled. For pulses having a duration less than about 35 fs, GDD, TOD, and FOD need to be controlled.
Changes in GDD, TOD, and FOD occur, for example, when adjustments are made to a regenerative amplifier, particularly the number of round-trips, in such an amplifier. Such changes may be made, for example, for changing the power of pulses output from the CPA. These must be corrected and controlled for optimum performance. Another example of changes in the GDD, TOD and FOD occurs when an experimental setup is adjusted and additional optical components are added in the beam between the compressor and the final target. Windows for sealing a vacuum-chamber, and focusing lenses used to focus a beam to a tight spot are typical examples of these added experimental components.
In prior-art solid-state CPA apparatus, control of GDD is accomplished by adjusting the spacing between reflection gratings in the pulse-compressor. Control of TOD is accomplished by adjusting the incidence angle of radiation on the reflection-gratings of the compressor.
A problem with varying the grating incidence angles is that these reflection-gratings cannot be used with incidence at the Littrow angle. The Littrow angle is an incidence angle on a grating which gives an equal diffracted angle. There must be some difference, however, between the incident and diffracted angles to allow for input to be separated from output without beam clipping. Practically, the minimum difference in incidence-angle from the Littrow angle is about 6°.
An additional problem with varying the reflection-grating incidence angles comes from the large angular variation suffered by the diffracted beam. A result of this is that if the incidence angle change is too great, the corresponding deviation angle can change sufficiently that other beam-steering optics, such as retro-reflectors, for providing multiple grating incidences, must be realigned laterally or horizontally, to intercept the deviated beam.
This means that, for maximum convenience and flexibility the pulse-compressor must be provided with at least one rotation-stage for rotating a grating, a translation stage for changing grating spacing and other translation means for realigning beam steering optics cooperative with the gratings. This results in apparatus which is massive, complicated, and expensive. There is a need for a simpler method and apparatus for controlling GDD and TOD in solid-state CPA apparatus.